<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
 <head>
  <meta http-equiv="content-type" content="text/html; charset=UTF-8">
  <title>自然对数</title>
<link media="all" rel="stylesheet" type="text/css" href="styles/03e73060321a0a848018724a6c83de7f-theme-base.css" />
<link media="all" rel="stylesheet" type="text/css" href="styles/03e73060321a0a848018724a6c83de7f-theme-medium.css" />

 </head>
 <body class="docs"><div class="navbar navbar-fixed-top">
  <div class="navbar-inner clearfix">
    <ul class="nav" style="width: 100%">
      <li style="float: left;"><a href="function.log1p.html">« log1p</a></li>
      <li style="float: right;"><a href="function.max.html">max »</a></li>
    </ul>
  </div>
</div>
<div id="breadcrumbs" class="clearfix">
  <ul class="breadcrumbs-container">
    <li><a href="index.html">PHP Manual</a></li>
    <li><a href="ref.math.html">Math 函数</a></li>
    <li>自然对数</li>
  </ul>
</div>
<div id="layout">
  <div id="layout-content"><div id="function.log" class="refentry">
 <div class="refnamediv">
  <h1 class="refname">log</h1>
  <p class="verinfo">(PHP 4, PHP 5, PHP 7, PHP 8)</p><p class="refpurpose"><span class="refname">log</span> &mdash; <span class="dc-title">自然对数</span></p>

 </div>
 <div class="refsect1 description" id="refsect1-function.log-description">
  <h3 class="title">说明</h3>
  <div class="methodsynopsis dc-description">
   <span class="methodname"><strong>log</strong></span>(<span class="methodparam"><span class="type">float</span> <code class="parameter">$num</code></span>, <span class="methodparam"><span class="type">float</span> <code class="parameter">$base</code><span class="initializer"> = <strong><code>M_E</code></strong></span></span>): <span class="type">float</span></div>

   <p class="para rdfs-comment">
    如果指定了可选的参数 <code class="parameter">base</code>，<span class="function"><strong>log()</strong></span>
    返回 log<sub class="subscript">base</sub>
    <code class="parameter">num</code>，否则 <span class="function"><strong>log()</strong></span> 
    返回参数 <code class="parameter">num</code> 的自然对数。
   </p>
  </div>

  <div class="refsect1 parameters" id="refsect1-function.log-parameters">
   <h3 class="title">参数</h3>
   <p class="para">
    <dl>
     
      <dt>
<code class="parameter">num</code></dt>

      <dd>

       <p class="para">
        要计算对数的值
       </p>
      </dd>

     
     
      <dt>
<code class="parameter">base</code></dt>

      <dd>

       <p class="para">
        可选的底数（默认是“e”，也可以说是自然对数）。
       </p>
      </dd>

     
    </dl>

   </p>
  </div>

 <div class="refsect1 returnvalues" id="refsect1-function.log-returnvalues">
  <h3 class="title">返回值</h3>
  <p class="para">
   以 <code class="parameter">base</code> 为底
   <code class="parameter">num</code> 的对数，如果未指定
   <code class="parameter">num</code> 则为自然对数。
  </p>
 </div>

 <div class="refsect1 seealso" id="refsect1-function.log-seealso">
  <h3 class="title">参见</h3>
  <p class="para">
   <ul class="simplelist">
    <li class="member"><span class="function"><a href="function.log10.html" class="function" rel="rdfs-seeAlso">log10()</a> - 以 10 为底的对数</span></li>
    <li class="member"><span class="function"><a href="function.exp.html" class="function" rel="rdfs-seeAlso">exp()</a> - 计算 e 的指数</span></li>
    <li class="member"><span class="function"><a href="function.pow.html" class="function" rel="rdfs-seeAlso">pow()</a> - 指数表达式</span></li>
   </ul>
  </p>
 </div>

</div></div></div></body></html>